A good fiberglass safe paint stripper is needed for the removal of the old finish and stain. Good products are available from Back to Nature, some examples are; Multi-Strip, Lift-N-Strip and Ready-Strip. Use the stripper according to instructions.
After the door is cleaned up you can use a <a href="//www.painters-toys.com/wood-stains/wood-gel-stain.html" target="_blank">gel stain</a> and good varnish or <a href="//www.painters-toys.com/varethane-exterior-water-based-finish-satin.html" target="_blank">water based urethane</a>.
Check out these instructions for the details; <a href="https://www.house-painting-info.com/staining-a-fiberglass-door.html">Staining a Fiberglass Door</a>.